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Biogas and fermentation product manufacturing process

The process in which biomass is converted into biogas also occurs in nature, take marshlands for instance. However, the organic material for the ReFood biogas plants is comprised entirely of food waste.

How does this process work? Compare it with placing a pan of split pea soup in the sun - after a period of time it will start to ferment. The gases will escape (these are then captured and converted into energy and heat) after which a concentrated composition remains. This composition is, of course, entirely hygienised (free from animal bacteria that are harmful to human health) and can be used directly in agriculture.

ReFood Biogas concept

1 | Storage silo for hydrolysis

The prepared biomass first arrives in the biogas plant in a storage silo. There, the carbohydrates, protein and fats are split into simple organic nutrients, generally long chain fatty acids. This process is known as hydrolysis.  Different bacteria strains are each responsible for processing certain substances. This pre-acidification reduces the pH value to around 3.5 to 4.

2 | Digester

The most important fermentation process takes place in the digester, in which the organic mass is converted into biogas, which essentially comprises methane and carbon dioxide. This process takes place through the exclusion of light and oxygen.

The long chain fatty acids are first formed into short chain fatty acids through hydrolysis, from which only the so-called methane-forming bacteria generate the biogas. This biogas is comprised of 60% methane. In this part of the biogas production, constant conditions in terms of temperature, pH value and fill capacity are needed to achieve optimal bacteria activity.

3 | Secondary fermentation silo/Combined storage

The main fermentation process produces two products from the biomass: biogas and a fermentation product that is used as a liquid fertiliser. Further fermentation takes place in the combined storage to split organic waste that is difficult to decompose.

4 | Gas processing: Desulphurising, cooling, cleaning

Before the biogas is used in the cogeneration unit, ReFood desulphurises it to protect the motor, gas catalyser and steam boiler.   Bacteria convert the hydrogen sulphide into elemental sulphur in a biological desulphurisation unit.

The fermentation product is then added as high-quality nutrient for plants.

5 | Gas storage facility

As the biogas yield is not always constant, a storage facility acts as a buffer for an even gas supply. The gas is supplied from the gas storage facility via pipelines to a gas cooler and dryer to separate the relatively humid gas from the condensate (water). The active carbon filter removes the remaining hydrogen sulphide from the gas.

6 | Cogeneration

In the cogeneration unit a gas motor burns the biogas to drive a generator, which in turn generates electricity. ReFood uses part of this energy for its own production processes at its plants. The remaining electricity is supplied to the public grid via a transformer. The released heat from the central heating plants is used for hot water and such things as vapour extraction. In the event of maintenance or a malfunction, for safety reasons it is possible to burn the gas using an emergency flare. 

7 | Storage of fermentation product

After the final degassing in the combined storage, an organic fertiliser, rich in minerals and trace elements remains. Agricultural service providers take the fermented product for use as fertiliser on farm and grassland. During the legally prescribed blocking period, the fermentation product is stored until spring.

8 | Control room
9 | Transformer station and connection to the national grid

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